1. Field of the Invention
The present invention relates to a method of producing a diamond coating on an object by means of a reactive plasma enhanced deposition process.
It also relates to a vacuum treatment apparatus for the production of diamond coatings including a vacuum chamber, an inlet arrangement which opens thereunto for introducing a process gas which is brought to react at least partly in the chamber, and a draw-off arrangement for gaseous reaction product.
It relates further to a method of setting the temperature at a supporting element for an object to be treated, i.e., workpiece to be diamond coated in a plasma coating chamber.
2. Description of the Prior Art
In recent years, many methods of diamond coating have been proposed and tested. Hereto, reference is made to the general paper of S. Matsumoto, "Proceedings First International ECS Symposium on Diamond and Diamond-Like Films", Los Angeles, 1989.
The various known methods incorporated such severe problems that to the belief of the inventor until now, nobody has succeeded in designing apparatus which provides a diamond coating on a large technical scale. In many of the known methods, a glowing filament located close to the substrate is used. Heretofore, extremely thin wires having diameters &lt;1 millimeter must be positioned at a distance of 1 to 2 centimeters from the surface of the object to be coated and maintained at about 2000.degree. C.
Apart from the fact that such a coating technique for spacious surfaces such as of drawing dies or tool punches poses immense problems, the useful lifetime of the above-mentioned glowing coils or filaments is short because of their being subjected to the reaction process.
Also in methods where the generation of the plasma is made by means of microwaves or RF-discharges, problems occur specifically when larger surfaces, for instance, surfaces larger than 400 square millimeters, are to be diamond coated.
When using direct voltage glow discharges for the generation of plasma, these techniques lead to known and often proven problems that it is not, or only with great difficulties, possible to achieve a uniform density of the plasma on surfaces of three-dimensionally formed bodies. For the production of diamond coatings, their quality depends, however, very critically upon the distribution of the density of the plasma in the immediate neighborhood of the surface to be coated.
U.S. Pat. No. 4,851,254 discloses a method and apparatus for the production of diamond coatings. A direct voltage arc discharge is generated in a vacuum chamber across an anode/cathode space for the production of a plasma and the object to be treated is located outside of the cathode/anode space. The process gas for the reactive plasma enhanced coating process is fed in at distinct points and opposite the object to be treated with respect to the cathode/anode space.
Due to the displacement of the object with respect to the cathode/anode space and thus with respect to the maximum of density of plasma therein, the object may be maintained at lower temperatures such that a temperature of the object being treated at 800.degree. C. can be installed during the coating only by applying additional heating.
The arc discharge occurs at low voltages due to the small anode/cathode distance at about 20 volts and at high currents on the order of about 40 amps. The object being treated is, however, arranged considerably distant from and outside of the plasma of a high power density generated by such an arc.
Thus, the plasma density produced by such procedure is not at all exploited for the coating process. The process is carried out at about 3500 PA process atmospheric pressure.
The drawback of this arrangement is that in spite of the high pressure, only an extremely short anode/cathode gap and a correspondingly short plasma space is used and that the larger object being treated located outside of the discharge gap or space is coated homogeneously only at a small area, or must be located at such a distance from the almost point-like discharge gap that it must be heated.
U.S. Pat. No. 4,859,490 discloses, furthermore, producing diamond coatings by a plasma enhanced, reactive deposition in that a glowing coil or filament is arranged in a vacuum chamber which relative to a grid, is connected to a cathodic DC-potential. Opposite the grid with respect to the glowing coil a support for the objects to be treated is provided, which, relative to the anodically-operated grid, is connected to a cathodic potential.
Process gas is led centrally along the glowing coil such that the plasma is produced in the cathode/anode space which is the glowing coil and the grid. Because the object between the cathodically-driven support is located outside of the anode/cathode space and, thus, again outside of the area of highest plasma density, low treatment temperatures between 600.degree. C. and 1200.degree. C. result.
The process pressure is stated to be between 700 and 28,000 Pa.
This procedure has the following drawbacks:
As already mentioned initially, also here the glowing coil is exposed to the process gas which on the one hand reduces its useful lifetime considerably and on the other hand leads, furthermore, to a cathode surface which is small regarding its extent, such that the conical expansion of the plasma occurs only due to the anodic grid with a corresponding decrease of the plasma density, specifically also at the location of the object being treated.
The article "Diamond Film Preparation by Arc Discharge Plasma Jet, Chemical Vapor Deposition in the Methane Atmosphere" of Naoto Ohtake and Masanori Yoshikawa, J. Electrochem. Soc., Vol. 137, No. 2, February 1990, discloses a process of diamond coating by the aid of a plasma beam. A gas is jetted at supersonic speed out of a nozzle cathode and the plasma is produced in the cathode/anode space having the anode extending perpendicularly to the cathode, and is led due to the jet impact linearally towards the object being treated. The object is cooled intensively because it otherwise would melt due to the high plasma jet temperature.
Apart from the cooling which must be foreseen, this procedure necessitates large expenditures for the generation of the plasma jet, and due to the restricted extent of the plasma jet only relatively small surface areas of the object can be coated.
The article "Diamond Synthesis by Hollow Cathode Plasma Assisted Chemical Vapor Deposition" of B. Singh, O. R. Mesker et al., Spie Vol. 877 Micro-Optoelectronic Materials (1988) discloses production of a diamond coating by means of plasma enhanced reactive coating in a vacuum chamber by producing between a hollow cathode and an anode grid or screen a plasma cone similar to the procedure according to U.S. Pat. No. 4,859,490 and arranging the object to be coated opposite the anode grid with respect to the cathode and connecting it to anode potential.
Apart from the fact that here, in contrast to U.S. Pat. No. 4,859,490, no glowing coil is used, with the drawbacks explained in connection with the above-mentioned publication, this procedure incorporates the same drawbacks as concerns the density of the plasma. Because the object to be treated is connected to anode potential, it must be cooled in case of high discharge currents.